1. Field of the Invention
The present invention relates to data storage and processing technology, for example to the replication of data among a plurality of storage subsystems.
2. Description of the Related Art
In the past, a technique in which data stored in a first storage subsystem are replicated in second and third storage subsystems has been considered so that when a fault occurs in the first storage subsystem, a data processing system comprising the first storage subsystem can continue to provide service. An example of this type of technique is disclosed in Japanese Unexamined Patent Application Publication 2003-122509. According to this technique, a first storage subsystem holds a first logical volume, a second storage subsystem holds a second logical volume, and a third storage subsystem holds a third logical volume. When the first storage subsystem writes data (to be referred to as “data A” hereafter) into the first logical volume, the data A are transferred to the second storage subsystem and written into the second logical volume synchronously with the timing at which the data A are written into the second logical volume, and the data A are transferred to the third storage subsystem and written into the third logical volume asynchronously with the timing at which the data A are written into the first logical volume.
Furthermore, the system described below, disclosed in U.S. Pat. No. 6,209,002, is known as an example of a data processing system comprising three or more storage subsystems.
A first storage subsystem holds a first logical volume serving as a replication source. A second storage subsystem holds a second logical volume serving as the replication destination of the first logical volume, and a third logical volume serving as the replication source of the second logical volume. A third storage subsystem holds a fourth logical volume serving as the replication destination of the third logical volume. When the first storage subsystem performs a data update on the first logical volume serving as the replication source, the data in the second logical volume of the second storage subsystem are updated. The second storage subsystem executes data replication processing from the second logical volume to the third logical volume and data replication processing from the third logical volume to the fourth logical volume exclusively.
In the prior art which is Japanese Unexamined Patent Application Publication 2003-122509, a function for learning the state of progress of data transfer, reception, and update between the second storage subsystem and third storage subsystem is provided in each of the second storage subsystem and third storage subsystem. In certain cases, however, it is desirable to perform replication between the second and third storage subsystems while maintaining data consistency without providing such a function.
Furthermore, in the prior art which is U.S. Pat. No. 6,209,002, upon reception of a data write command, the first storage subsystem writes data into both the first logical volume and the second logical volume in the second storage subsystem, and hence the response time to the write command may lengthen depending on the physical distance between the first storage subsystem and second storage subsystem. To reduce the response time to a write command, the physical distance between the first storage subsystem and second storage subsystem must be shortened.